This invention relates to an infrared sensor and, more particularly, it relates to a signal read circuit of a thermal type infrared sensor.
The technology of infrared imaging can find a broad scope of application because images can be picked up regardless of day or night even through smoke and fog to provide a great advantage over visible light imaging. Additionally, infrared imaging can obtain thermal information on the object of imaging. The broad scope of application of infrared imaging covers military defense devices, monitor cameras and fire detection cameras.
In recent years, massive efforts have been paid for developing no cooling thermal type infrared solid state imaging devices that do not require the use of a cooling mechanism for low temperature operations because the need of using a cooling mechanism is the largest problem of quantum type infrared solid state imaging devices that have been in the mainstream. Thermal type infrared solid state imaging devices are so designed that the incoming infrared rays with a wavelength of about 10ìm are transformed into heat by means of an IR rays absorption system and the temperature change of the heat sensing section produced by the weak heat is converted into an electric signal by means of a thermoelectric converter. Then, infrared image information can be obtained by reading the electric signal.
Thermal type infrared solid state imaging devices realized by forming silicon pn junctions for converting a temperature change into a voltage change by means of a constant forward electric current in an SOI (silicon on insulator) region have been reported (Tomohiro Ishikawa, et al., Proc. SPIE Vol. 3698, p. 556, 1999).
Silicon pn junction type devices using an SOI substrate provide an advantage that they can be manufactured by using only a silicon LSI manufacturing process and hence are highly adapted to mass production.
Another advantage of silicon pn junction type devices is that the pn junctions that operate as thermoelectric conversion means have a pixel selecting function of utilizing the current rectifying ability of pn junctions and therefore it is possible to simplify the internal structure of pixels.
Meanwhile, the temperature change in the pixel section of a thermal type infrared solid state imaging device is generally about 5xc3x9710xe2x88x923 times of the temperature change of the object of imaging although it may vary depending on the absorption coefficient of the infrared rays absorption layer and the performance of the optical system. In other words, when the temperature of the object of imaging changes by 1[K], the pixel temperature changes by 5[mK].
When eight silicon pn junctions are connected in series to a single pixel, the thermoelectric conversion efficiency is in the order of about 10[mV/K]. Therefore, when the temperature of the object of imaging changes by 1[K], a signal voltage of 50[ìV] is generated in the pixel section.
In reality, the thermal type infrared solid state imaging device is required to detect a temperature change of about 0.1[K]. Then, it has to read a generated voltage signal of about 5[ìV].
As means for reading such a very weak signal voltage, a circuit adapted to amplify the generated signal voltage as the gate voltage of a MOS amplifier transistor for amplifying the electric current and integrate the amplified current with time by means of a storage capacitor is known.
Such a circuit is referred to as gate modulation integration circuit. An effect of limiting the signal bandwidth and reducing the random noise can be achieved by arranging such a circuit as column amplifying circuit in each column of a matrix for the purpose of parallel amplification of the electric current of a row.
The voltage gain: G of a gate modulation integration circuit is determined by the mutual conductance of the amplifier transistor: gm=xc3xa4Id/xc3xa4vg, the integration time: ti and the storage capacity Ci and expressed by G=(tixc3x97gm)/Ci. When the integration time: ti and the storage capacity: Ci are given, the gain is dominated by the mutual conductance: gm of the amplifier transistor. The value of gm is approximately expressed by formula (1) below when an n-type MOS transistor operates in a saturation region;
gm=(W/L)xc2x7({dot over (a)}ox/Tox)xc2x7ìnxc2x7(Vgsxe2x88x92Vth)xe2x80x83xe2x80x83(1),
where W: channel width,
L: channel length,
{dot over (a)}ox: dielectric constant of gate oxide film,
Tox: film thickness of gate oxide film,
ìn: electron mobility,
Vgs: gate/source voltage and
Vth: threshold voltage of transistor.
As pointed out above, a thermal type infrared solid state imaging device is required to detect a temperature change of about 0.1[K] in the temperature of the object of imaging. Therefore, it is necessary to read a signal of about 5[ìV] that is generated in the pixel section. This voltage level is very low if compared with a CMOS sensor that is used to obtain an image by means of visible light. According to Nakamura and Matsunaga, xe2x80x9cHigh Sensitivity Image Sensorxe2x80x9d, the Journal of the Institute of Image Information and Television Engineers, Vol. 54, No. 2, p. 216, 2000, the noise voltage is about 0.4[mV]=400[ìV]. In view of this noise level, the noise level of the above infrared solid state imaging device is as low as about 1/80 of that of a CMOS sensor and hence the signal voltage the former deals is as low as about 1/80 of that of the latter.
Therefore, if the sensor output is processed by means of a circuit similar to a circuit to be for processing the output of a CMOS sensor that is a typical imaging device, a column amplifier comprising gate modulation integration circuits and showing a gain of about 80 times will be required.
However, a variance greater than the pixel output of about 5[ìV] is found at the gate of the amplifier transistor of a gate modulation integration circuit and hence such a circuit needs to be designed with a relatively low gain. The variance is attributable to the variance of the threshold voltage of the MOS amplifier transistors and that of the threshold of the load MOS transistors to be used as constant current source and it is known that both show an amplitude of about 30[mV].
The amplitude of the fluctuations of the threshold voltage means that the storage capacity can show fluctuations of about 2.4[V] when an about 80 times greater gain is used as design value because the fluctuations are amplified by the amplifier/read circuit like the pixel output signal applied as the gate voltage of the amplifier transistor. Of course, the fluctuations of the thresholds are specific to the individual MOS amplifier transistors and the individual load MOS transistors and a fixed pattern appears in the picked up image so that the obtained image can be corrected by means of an external circuit. However, such corrections use most of the voltage swing of the storage capacitor and expand the dynamic range that the external circuit is required to show.
Therefore, until now, the load applied to the external circuit has to be inevitably reduced at the cost of the gain of the amplifier/read circuit. Furthermore, it has not been possible to sufficiently suppress random noises such as current shot noises and 1/f noises of the amplifier/read circuit in order to secure a large gain.
Additionally, in many cases, an electric current has to be made to flow to the thermoelectric converter of the thermal type infrared sensor in order to read the thermal information of the thermoelectric converter as electric signal. Then, a so-called self heating problem arises because Joule""s heat is generated due to the bias current or the bias voltage to be used for reading the thermal information and the generated Jole""s heat by turn heats the thermoelectric converter.
For instance, when thermoelectric conversion pixels are mounted onto a semiconductor substrate and the general value of 10xe2x88x927[W/K] is selected for the thermal conductance between the semiconductor substrate and the pn junction type thermoelectric converter, the influence of self heating of the converter will be a temperature rise of about 30[K] if computed on an assumption that the number of pn junctions is eight, the bias current is 200[ìA], the pixel selection period for signal reading is 25[ìs] and the frame rate is 60[fps]. The importance of solving the problem of self heating will be realized if the above value is compared with the pixel temperature rise of 5[mK] due to the received infrared rays described earlier.
FIG. 26 of the accompanying drawings schematically illustrates the temperature change (in terms of voltage Vsig) of a pixel due to self heating. As shown, the pixel temperature rises rapidly as the pixel is selected in a row selection period and falls gradually after the pixel selection pulse becomes off due to the thermal time constant of the thermoelectric converter.
Thus, the temperature change due to self heating is 30[k] according to the above computation while the temperature signal generated by the incoming infrared rays that give rise to a temperature change of only about 5[mK] is smaller than the height of the solid line in FIG. 26. Thus, in the case of an ordinary column amplifier connected to a signal line, a weak electric current flows in the initial stages of pixel selection and the signal current increases with time due to self heating during the pixel selecting operation. Additionally, the electric current is almost totally occupied by the temperature information current produced by the self heating, which is a mere noise current.
FIG. 27 of the accompanying drawings schematically illustrates the integrated and stored electric charge in a storage capacitor that is depicted as a potential well and located at the output side of the column amplifier. As seen from FIG. 27, the stored electric charge is mostly the electric charge QSH that is attributable to self heating and the signal charge Qsig is only a minor part thereof.
FIG. 27 also shows that the electric current generated in a latter half of the row selection period becomes relatively large as a result of the temperature change caused by self heating and consequently the information obtained in the latter half and in the final stages of the row selection period is weighted. Then, as a matter of course, the effective sampling period is curtailed to expand the signal bandwidth and increase random noise.
X. Gu, et al. reports a method for avoiding the problem of self heating by forming a bridge circuit, using a bolometer operating on the principle of temperature change of electric resistance (X. Gu, et al., Sensors and Actuators A, Vol. 69, p. 92, 1998).
The authors of the above document formed a bridge circuit to realize differential amplification by arranging an insensitive reference pixel having a heat capacitance same as that of the ordinary pixels and showing a low thermal resistance at each column.
This is a method utilizing the fact that the temperature rise due to self heating in the row selection period that is very short relative to the thermal time constant is mainly governed by the heat capacitance.
However, while this method is effective for reducing the influence of self heating, it is an approximate solution to the problem of self heating and does not completely dissolve the self heating problem.
Insensitive reference pixels have to be provided on a one to one basis relative to the sensitive pixels for the purpose of completely dissolving the self heating problem in a rigorous sense of the words. However, no image sensor having a layout of arranging pixels two-dimensionally has ever been realized to date.
This is because, when an insensitive reference pixel is provided for each ordinary pixel in order to form a bridge circuit, there arises a disadvantage that the sensitivity of the image sensor is reduced to less than a half if the pixel size of the reference pixel is same as that of an ordinary pixel. Therefore the effect of dissolving the problem of self heating is offset by this disadvantage and hence such a technique of canceling self heating by means of a bridge circuit may not be effective.
In view of the above identified circumstances, it is therefore an object of the present invention to provide a low noise, high sensitivity and wide dynamic range uncooled type infrared sensor that can effectively reduce the influence of fluctuations of the gate of the amplifier transistor and a method of driving such an infrared sensor.
The self heating problem of pn junction type infrared sensors comprising thermoelectric conversion pixels having a column amplifier has not been dissolved in the technological field of the present invention. Therefore, another object of the present invention is to solve this problem.
In an aspect of the present invention, there is provided an infrared sensor comprising:
an imaging region containing thermoelectric conversion pixels arranged two-dimensionally in the form of a matrix of a plurality of row and a plurality of columns on a semiconductor substrate to detect incident infrared rays;
a plurality of row selection lines arranged in the column direction in the imaging region;
a plurality of signal lines arranged in the row direction in said imaging region;
a plurality of amplifier transistors having respective gates connected to said signal lines and configured to be modulated by the respective signal voltages generated in the signal lines;
a plurality of storage capacities connected respectively to the drains of the amplifier transistors and configured to store signal charges from the transistors;
a plurality of reset circuits connected to the respective drains of said amplifiers to reset the drain potentials of said amplifier transistors and make them show a predetermined potential;
a plurality of read circuits for reading the respective signal charges stored in said storage capacities;
a plurality of coupling capacities arranged respectively between said signal lines and the gates of said amplifier transistors; and
a plurality of sampling transistors connected respectively between the drains and the gates of said amplifier transistors to selectively turning them on to apply the threshold information of said transistors to the gates.
Some of modes of carrying out the invention includes the following.
(1) Each of the thermoelectric conversion pixels comprises an infrared absorbing section for absorbing infrared rays striking the semiconductor substrate and converting them into heat; a thermoelectric conversion section for converting the temperature change produced by the heat generated in the infrared absorbing section into an electric signal, a pixel selection circuit for selecting a pixel to be used for reading the pixel output signal from the thermoelectric conversion section and an output circuit for outputting the pixel output signal from the selected thermoelectric conversion pixel.
(2) The semiconductor substrate is an SOI substrate SOI formed by laying an SOI single crystal silicon layer on a single crystal silicon supporting substrate by way of a silicon oxide layer and the thermoelectric conversion means comprises single crystal silicon pn junctions formed by arranging second conductivity type regions in the inside of the first conductivity type SOI single crystal silicon layer and supported on the cavities of the cavity structure formed in the inside of the SOI substrate.
(3) The imaging region contains at least a row of insensitive pixels showing no sensitivity relative to incident infrared rays and hence being not apt to change any pixel output signal by incident infrared rays, said insensitive pixels being arranged in the row direction.
(4) The insensitive pixels are made insensitive as the thermoelectric conversion means is not thermally separated from the semiconductor substrate.
(5) The infrared sensor further comprises a storage means for storing a first group of pieces of row output information obtained from the read means on a time series basis and a correction means for correcting a second group of pieces of row optical information obtained by selecting a row different from the row used for obtaining the first group of pieces of row output information.
An infrared sensor having the above configuration is typically driven in a manner as described below. During a non-selection period when any of said thermoelectric conversion pixels is selected by means of said row selection lines in a single frame period, the drain potentials of the amplifier transistors are held in an unreset state by turning off said reset means and the drains and the gates of the transistors are connected and made to show a same potential by turning on said sampling transistors in the first period of the non-selection period. In a second period of said non-selection period not including the first period, the drains and the gates of the sampling transistors are separated form each other by turning off said sampling transistors and the gates of said amplifier transistors are made to hold the drain potentials of the respective transistors obtained in the first period. During a selection period for selecting a thermoelectric conversion pixel by means of said row selection lines and a period for reading the signal voltages by said read means, the sampling transistors are held to an off state.
An infrared sensor comprising an insensitive pixel row is typically driven in a manner as described below.
During a first selection period when the insensitive pixel row is selected by means of said row selection lines, the drain potentials of the drains of said amplifier transistors are held to an unreset state by turning off said reset means and the drains and the gates of said sampling transistors are connected and made to show a same electric potential by turning on said sampling transistor to apply a first source voltage to the sources of said amplifier transistors in a first period of said first selection period. In a second period of said selection period not including the first period, the drains and the gates of the sampling transistors are separated form each other by turning off said sampling transistors and the gates of said amplifier transistors are made to hold the drain potentials of the respective transistors obtained in the first period. During a second selection period for selecting a sensitive pixel row by means of said row selection lines in a single frame period, said sampling transistors are held to an off state and a second source voltage different from the first source voltage is applied to the sources of said amplifier transistors. In a period for reading the signal voltage of the selected amplifier transistor by means of said row selection lines by said read means, said sampling transistors are held to an off state and the first source voltage is applied to the sources of said sampling transistor.
Thus, according to the present invention, the signal lines where the pixel output appears and the gates of the amplifier transistors are separated from each other DC-wise by arranging coupling capacitors between them and each frame is made to hold the threshold information of the amplifier transistors of each column in the gates of the amplifier transistors in order to eliminate any variance among the voltage gains of the columns that can be produced as a result of fluctuations of the threshold value of the amplifier transistors, which may vary from column to column. Thus, the influence of fluctuations of the threshold value of each column can be eliminated to make it no longer necessary to provide a margin for the operating voltage region of the storage capacitors for the purpose of coping with such fluctuations and securing a voltage swing of the storage capacitors. Therefore, according to the invention, it is possible to design a large gain for the gate modulation integration circuit so that a highly sensitive uncooled type infrared sensor can be realized. Additionally, the potential of the storage capacitor can be fully exploited for the same reason to make it possible to provide a uncooled type infrared sensor having a wide dynamic range.
Thus, according to the invention, fluctuations of the threshold voltage of the amplifier transistors and the variance of operation point are corrected at the same time by sampling the threshold information of the amplifier transistors of each column and also the threshold information of the load MOS transistors of each column simultaneously at a timing when an insensitive pixel voltage is generated in the signal lines at the time of selecting an insensitive pixel that contains threshold information of the load MOS transistors. Thus, the influence of fluctuations of the threshold voltage of the load MOS transistors of each column can be eliminated to make it no longer necessary to provide a margin for the operating voltage region of the storage capacitors for the purpose of coping with such fluctuations and securing a voltage swing of the storage capacitors. Therefore, according to the invention, it is possible to design a large gain for the gate modulation integration circuit so that a highly sensitive uncooled type infrared sensor can be realized. Additionally, the potential of the storage capacitor can be fully exploited for the same reason to make it possible to provide an uncooled type infrared sensor having a wide dynamic range.
In still another aspect of the invention, there is provided an infrared sensor comprising:
a plurality of thermoelectric conversion pixels arranged in the form of a matrix of a plurality of rows and a plurality of columns and configured to thermoelectrically transform the heat generated as a result of absorbing incident infrared rays and take it out as a change in the resistance;
a plurality of selection lines connected respectively to either the rows or the columns of said thermoelectric conversion pixels;
a plurality of signal lines connected respectively to either the columns or the rows, whichever appropriate, of said thermoelectric conversion pixels;
a pixel selection circuit for selectively applying a read voltage to said thermoelectric conversion pixels connected to said the selection lines on a selection line by selection line basis and causing said signal lines to generate a voltage output signal;
an output signal amplifying circuit having a first input section and a second input section, said first input section being connected to said signal lines, and configured to amplify the voltage output signal from said thermoelectric conversion pixels; and
a compensation voltage applying circuit connected to the second input section of said output signal amplifying means and applying a wave form voltage for canceling or reducing the voltage component contained in said voltage signal due to the resistance change component attributable to the self heating produced in said thermoelectric conversion pixels by said read current in synchronism with said read voltage.
With an infrared sensor in this aspect of the invention, a ramp or step waveform voltage that is synchronized with the pixel selection pulse is applied to the sources of the amplifying MOS transistors of the amplifying circuit to whose gates the voltage signal is input from the thermoelectric conversion section in order dissolve the self heating problem of the thermoelectric conversion section of a thermal type infrared sensor according to the invention that is attributable to the Joule""s heat generated as a result of pixel selection. As a result, the voltage attributable to self heating can be removed from the gate/source voltage: Vgs of the amplifier transistors to realize a low noise, high sensitivity and wide dynamic range uncooled type infrared sensor.
Additionally, according to the invention, it is possible to provide an insensitive pixel column formed by arranging a heat isolation insensitive pixel on each row. It is then possible to remove the voltage attributable to self heating from the gate/source voltage: Vgs of the amplifier transistors by applying a voltage produced by referring to the output voltage of the insensitive pixel column to the sources of said amplifier transistors as in the case of applying a ramp waveform voltage. Thus, it is also possible to realize a low noise, high sensitivity and wide dynamic range uncooled type infrared sensor.